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With a plethora of big  endorsements under his belt, Mitt Romney is turning his criticism of Newt Gingrich up several notches. But the former House Speaker has declined to take the bait in what could become a nasty fight, leading some to wonder whether Romney's strategy could backfire on him.

"I'm going to stay positive, I'm going to talk about how we solve the country's problems. And I have one opponent, Barack Obama, and that's how our campaign is going to keep moving forward," Gingrich said in South Carolina today in a clear rebuke to Romney's campaign strategy. "Others are allowed to do what they want to do."

The former governor's latest attack is targeted at Gingrich's initial criticism of Rep. Paul Ryan's Medicare plan, which would revamp Medicare so that beneficiaries receive a set sum based on their income instead of the federal government paying for every service.

"What he did do to Paul Ryan is a perfect example of irrational behavior that you do not want in a commander-in-chief," former New Hampshire Gov. John H. Sununu, the latest political bigwig to endorse Romney, said in a scathing conference call this morning.

Earlier this year, Gingrich dubbed Ryan's plan " right-wing social engineering" and said that he doesn't think "imposing radical change from the right or the left is a very good way for a free society to operate."

Amid a backlash from conservatives, Gingrich later apologized publicly as well as privately to the Wisconsin congressman, saying he "made a mistake."

Romney himself hasn't been immune on the issue of health care. Despite his repeated calls to repeal the Affordable Care Act, it's hard for many conservatives to overlook the Massachusetts health care plan, which Romney signed and which became the basis for Democrats and President Obama's national health care law.

As he ramps up the attack on Gingrich - beginning with a not-so-subtle personal ad Wednesday - the SuperPAC that's supporting Romney officially launched a $3.1 million ad campaign in Iowa. Restore our Future, which boasts an impressive fundraising figure of $12.2 million, said the 30-second television ad will be targeted at President Obama's inexperience.

Gingrich, who is enjoying his lead in three out of four early-voting states, continues to focus on attacking Obama.

"I'm not going to pick a fight over Mitt Romney," he  told CNN Wednesday.

When reporters asked the former congressman from Georgia today whether the attacks are getting too personal, Gingrich responded, "No, he's fine."

A CNN/Time poll released Wednesday showed Gingrich leading Romney in Iowa, South Carolina and Florida. The Quinnipiac University poll released today showed Gingrich boasting a big lead in Florida, Ohio and Pennsylvania.

The former Speaker might have more than a few skeletons in his closet, but he's high among Republican voters when it comes to two main traits: authenticity and competence.

"You have someone like Mitt Romney, whose viewed as very competent for president but not authentic in his beliefs, and now you have someone in the rise of Newt Gingrich whose viewed as both authentic in his beliefs and competent in his ability to be president," Republican strategist and ABC News consultant Matthew Dowd said.

Being high on both those traits "puts him in the commanding lead so far."

The ABC News-Washington Post poll found Gingrich and Rep. Ron Paul leading Romney by 2-1 margins as the candidates most likely to "stand up for what he or she believes." Both similarly led Romney on empathy, as the candidate who "best understands the problems of people like you."

Breaking in on his chief opponent on those two tests is a tough challenge for Romney.

By going on an attack, Romney faces the risk of a backlash from Republican voters, said ABC News' Political Director Amy Walter. On the other hand, waiting for Gingrich to make a blunder could also backfire, considering there are only a few weeks left to the Iowa caucus.

ABC News' John Berman contributed to this report.